The present invention relates to an improved thermosensitive recording material comprising a support on which (i) a first thermosensitive coloring layer comprising a first colorless or light-colored leuco dye and a first color developer capable of inducing color formation in the first leuco dye upon application of heat thereto, and (ii) a second colorless or light-colored leuco dye and a color developer capable of inducing color formation in the second leuco dye upon application of heat thereto are overlaid in any order.
Recording materials using leuco dyes are conventionally known and used in practice, for example, as pressuresensitive recording sheets and thermosensitive recording sheets. A conventional thermosensitive recording material using such leuco dye is composed of a support and a thermosensitive coloring layer comprising a leuco dye and a color developer formed thereon. Colored images are formed on the thermosensitive coloring layer upon image-wise application of heat through a thermal resistor element to which image signals are applied.
Such thermosensitive recording materials are employed in a variety of fields, for instance, for use with printers of computers, recorders of medical analytical instruments, facsimile apparatus, automatic ticket vending apparatus, and thermosensitive copying apparatus, since they have such advantages over other conventional recording materials that (1) images can be formed by simple heat application, without complicated steps for development and image fixing, and therefore image recording can be speedily performed by a simple recording apparatus, without generation of noise and causing environmental pollution, and that (2) the thermosensitive recording materials are inexpensive.
In such conventional thermosensitive recording materials, there is usually employed in the thermosensitive coloring layer a thermal coloring system comprising a combination of (i) a colorless or light-colored leuco dye such as crystal violet lactone and leuco crystal violet which are colored in blue, and 7-anilino-substituted fluoran compounds which are colored in black, and (ii) a color developer which induces such color formation in the leuco dyes upon application of heat thereto.
In accordance with the recent development of optical character reading apparatus and bar-code reading apparatus, thermosensitive recording materials suitable for use in such reading apparatus are desired. In such reading apparatus, however, such a light source as emits light having a wavelength of 700 nm or more is in general use. However, the above-mentioned leuco dyes, when colored in blue or black, scarcely absorb light in a near infrared region, specifically light having a wavelength of 700 nm or more. Therefore, it is impossible for the above reading apparatus to read the characters or bar codes developed by the above leuco dyes.
Under such circumstances, there is a great demand for novel leuco dyes which absorb light having a wavelength of 700 nm or more when colored by a color developer.